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Quick question about sound holes:
In a guitar with multiple sound holes, assuming they are all circular, do the surface area of all the holes need to add up to about the same size as one big sound hole?

For the sake of argument, lets say a typical large bodied guitar has a sound hole that is 51mm
The area of said 51mm hole is about 8170 square mm

In order to break that up into smaller holes, that would be five circles with radii of 5, 5, 10, 15, 25, and 40mm (it's a fibanochi sequence and it works with the design of the guitar in question) Together, their radii add up to about 8170 square mm

All things aside, should a guitar with one 51mm sound hole sound 'the same' as one with the above mentionend radii?

Depending on responses, the follow up will be asked about whether the holes need to be in the top or not.

TIA for the help,

Dave

Considering the fact that moving the location of the soundhole changes it's effect on the sound of the guitar, it is doubtful that using multiple smaller holes adding up to the same surface area as a single hole would sound - exactly - the same. Depending on their location it would probably sound very similar, and still sound like a guitar.
Alan Carruth has worked with soundholes/ soundports in a more scientific way and might give you better guidance on the subject.

The size, or volume of the hole will affect the breathing mode of the instrument, the Helmholtz resonance. But moving the hole from being inline to the strings will change the way the instrument behaves. That central hole creates weakness, and some form of weakness is needed; isn't it? If the guitar doesn't give, what kind of sound can it make. Electrics are an entirely different thing.
Ladder braces, or x bracing, and all other forms of bracing are needed to keep the thin belly from squishing with the string tension; but the hole in the middle makes their job harder.
Violins and arch tops have f holes, at least most of them; and they purposely create weakness to allow the center area of the belly more, and less restricted movement. In this case, the belly has an arch that might, or might not be able to resist the string tension on its own, but 1 or 2 braces are added.
If the sound holes are tucked away somewhere safe, and out of the way; wouldn't you have to look more closely at your basic arching/bracing design? Wouldn't you have to drastically thin it down? If not, will it have much sound?
I don't know; I'm just asking.

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Why be normal?

I'm not sure the central hole creates weakness. I think most people reinforce the central hole and bring the heaviest braces adjacent so that area is actually stronger/stiffer????

But the hole IS addressed. If it wasn't there, then the bracing would not have to make up for the void.

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Why be normal?

Lets pretend I didn’t ask about guitars and the question was only about helmholz resonance.

Does the vibrating air mass resonate at a different frequency whether there is one hole in the middle of the face of the box, or multiple holes in various places around the face of the box, and those holes add up to the same area as the single big hole?

This could be a can of worms.

Think for a bit about a lute rose that's been cut into a top (not the parchment 'wedding cake' kind for the moment). It's a grid of holes that covers about the same area as an open sound hole. Obviously the actual open surface is smaller than the round hole that you get without the grid in place, but if you put such a grid in an open hole (say, one of those 'feedback busters') the actual air frequency doesn't drop as much as you'd expect from the reduction in area. What does change is the loss factor of the resonance; the added drag of the air flowing past all of those edges reduces the amplitude at the peak frequency, so the Q value is lower. The parchment type can actually have a larger open surface area than the round hole, because of the pierced sides of the layers, but it also reduces the Q more due to the (usually) larger amount of sharp edge. You can pretty effectively eliminate the 'main air' resonance with one of those, and that's how the feedback busters work: if the room can't hear the guitar at that pitch, the guitar can't hear the room, and you can use a lot more gain in the loop without getting feedback.

The location of the hole has a role in determining the 'air' resonant pitch as well. If you're thinking about something like the spread out arrangement of holes on an Ovation then I think you'd need to figure out the 'center of gravity' of the holes to get an idea of the effective location, and then reduce the Q value according to the added edge length.

My understanding is that a slot, such as the 'F' hole on a violin, has an equivalent area equal to a round hole about 1/3 the diameter of the slot's length, but I can't give you a reference to back that up. Again, the added edge length lowers the Q.

As you distribute the holes around a larger area you have to start paying attention to the internal air resonances that each one 'hears' that may not be picked up by a sound hole in the normal position. I once made a guitar with two smaller sound holes on either side of the upper bout. The UTB was converted into a shallow 'X', crossing at the usual spot at the end of the fingerboard, and the holes were out at the ends, just at the widest part of the upper bout. Since my bridge has an arc along the bottom it was a sort of 'smiley face', and I called in 'Caspar the Friendly Ghost'. The two holes 'heard' the crosswise air resonance in the upper bout (A-3?) and, being on opposite ends of the 'tube' they put that sound (at around 700 Hz or so) out 'out of phase' with each other. The two waves interfered in such a way as to reinforce at about the location of the player's head, so any note that had a partial at that pitch sounded strong to the player. The natural inclination was to back off on those notes, with the result that people out in front heard the guitar as 'uneven'.

The bottom line is that, as usual. there are to many variables involved to be able to generalize too far.

I'll note, since it came up, that the normal sound hole is less of a weak spot than it's often thought to be. As is usual with the top, stiffness count for more than strength, and it's fairly easy to stiffen up the area significantly without adding appreciable weight. I've got some evidence, going back about 20 years, that the 'normal' sound hole location is one of the things that helps make a guitar sound different from, say, an Irish bouzouki or cittern. Basically, there seems to be an interaction between the sound hole location, the waist, and the 'long dipole' resonance of the top that can 'split' the A-1 air resonance (the 'lengthwise bathtub' sloshing mode) into two, with one of them contributing significant sound output at the hole that would not happen otherwise. It's interesting that all three of these things; the outline, the hole location, and the top long dipole, seem to be 'strongly conserved' features, as an evolutionary biologist would say, in the guitar. Usually, when you see something like that, it's significant.

The Helmholtz frequency would be different because of the changes in the (aggregate) effective length of the "tube"
A discussion that explains it better than I can, is here:
https://newt.phys.unsw.edu.au/jw/Helmholtz.html

At the end is an explanation of how it relates to guitars which you may find interesting.

I'm confused as usual.
You say a typical large body guitar has a 51mm sound hole? (2") or are you saying 51mm radius sound hole? (4" diamater)

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Joe Beaver
Maker of Sawdust

From the area he mentioned , 51 is the radius. Pi* r (squared)= area
Pi * 51mm squared = 8170 sq.mm

Clay,
Thanks. That adds up.

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Joe Beaver
Maker of Sawdust

You're Welcome Joe,
In Courtnall's book, the plans show the soundhole's radius rather than it's diameter. Other books may do it this way also. It makes it easy to know what to set the circle jig at.

Thanks all for the responses.

The biggest take-a-way for me will be that with more than one sound hole, the helmholz resonator becomes an Ocarina.

Will proceed with that in mind.

dl

Maybe, just maybe, try contacting Chris Ensor on OLF, or even his "inspiration", Sheldon Schwarts to see if they can give some insights?
Lotsa holes, everywhere http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=51088&hilit=dream+guitar&start=25
The idea is interesting, and asymmetric placement should help avoid resonance problems.
Of course Ovation have done it too, though not in your specific sizes.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

And how will a Luthier in ten years access the inside of the guitar to reglue a brace when an accident happens or to repair a bridge plate that has worn pin holes or, or, or..... Even O*ation.... has allowed for access with a port on the back. Might be a good idea to consider how to keep the serviceability in place with any reduction of sound holes size too small for a human hand.

Might be a good reason to consider adding a tail port.

Oh Hesh. I’m hurt that you don’t think I’ve already thOught of that!

I’ve twice responded with details of what I have in mind. Both times, it took me so long to figure out how to post a picture, I gave up.

Carleen Hutchins made her 'La Gruyere' fiddle to investigate the interaction of air and wood resonances. She had 63 little holes all around the ribs, and could alter the air resonances in all sorts of ways depending on which ones she removed. With all of the corks in it was a pretty nice fiddle, but as you removed more and more of them the tone deteriorated. It was, of course, the inspiration for my own 'Corker' guitar, but I only used 20 holes, and only on the bass side. Still, the result was broadly similar; taking them all out really killed the sound.

I've never been particularly impressed with guitars that had large ports; they tend to have an uninteresting tone IMO, and too 'forward' a sound. Partly that's due to the high pitch of the 'main air' resonance.

The bottom line for me is that lots of holes don't make an ocarina, but they do make a poor guitar.

OK I stand corrected my friend

I put a great bit access port in just for you (though I hope you never have to.)